1. Field of the Invention
The present invention relates to an encoder, and particularly to an optical encoder in which an optical means is used.
2. Description of the Related Art
For detecting an amount of displacement in a straight line direction in a measuring instrument and a stage of a machine tool, or for detecting an angle of rotation in a servo motor etc, a so-called encoder of an optical type and a magnetic type has been hitherto used.
An optical encoder, in general, includes a scale which is fixed to a member such as the stage etc. of which the displacement is to be detected, and a sensor head for detecting a displacement of the scale. The sensor head includes a light emitting section which irradiates light to the scale, and a light receiving section for detecting a light beam which is modulated by the scale. Moreover, a movement of the scale is detected by a change in an intensity of the light beam which is received.
An optical encoder which has a function of detecting a reference position in addition to an amount of movement of the scale is hitherto known. The reference position is used for example, for detecting an origin of the stage etc, or for detecting a critical point of movable range. Therefore, at the time of detecting the reference position, it is sought that there is no wrong detection, and that position repeatability is satisfactory.
As a method of detecting the reference position, a number of methods have been hitherto proposed (Japanese Patent Application Laid-open Publication No. Hei 8-178613 for example). FIG. 21 shows a schematic structure of an optical encoder 10 of a conventional technology. In the optical encoder 10 of the conventional technology, light emitted from a light source is irradiated to the scale (not shown in the diagram). Further, light reflected from the scale is detected by two cell-photodetectors (PD) 11 and 12. Here, the optical encoder 10 has a reference-position detecting function.
The light irradiated from the light source is focused by a reference-position pattern formed on the scale which is not shown in the diagram. Light from the reference-position pattern forms a beam spot 20 on the two cell-photodetectors 11 and 12. Detection signals 11a and 12a output from the two cell-photodetectors 11 and 12 respectively are output to a processing circuit which is not shown in the diagram. The beam spot 20 moves on the two cell-photodetectors 11 and 12 with the movement of the scale, in left and right direction in the diagram which is a direction of movement of the scale. Therefore, when a difference between the output signals 11a and 12a from the two cell-photodetectors 11 and 12 is calculated, a resultant signal is as shown in FIG. 22. As a result of this, a position 50 at which a differential signal performs a zero crossing can be detected as the reference position. Moreover, a photodetector 40 is used for detecting the movement of the scale.
The differential signal for detecting the reference position is a difference between electric outputs from the two cell-photodetectors 11 and 12. Therefore, the differential signal becomes the zero crossing, or the same electric potential thereof, when the beam spot 20 is incident evenly on the two cell-photodetectors 11 and 12, and when the beam spot is not irradiated to either of the cell-photodetectors 11 and 12.
In other words, as shown in FIG. 22, the point 50 (position 50) of zero crossing and points 51a and 51b of zero crossing at both end portions in FIG. 22 cannot be distinguished. Therefore, there is a possibility that in both cases, namely in a case of point 50 being the zero crossing and in a case of other points 51a and 51b being zero crossing, may be detected as the reference position. Particularly, in a stationary state, it is difficult to distinguish the points of zero crossing. In the conventional technology, a judgment of whether or not being the reference position cannot be made without taking an excessive trouble such as moving the scale intentionally to left and right.
Due to such problem, in the abovementioned machine tool, the movement to the reference position, or detection of a desired position in which the reference position is let to be a base becomes inaccurate. Furthermore, due to a defect of processed (finished) good, and a mistake of a working position, there is a possibility of occurring of damage to the apparatus itself.